1. Field of the Invention
This invention relates to waveguide coupling networks and in particular, but not exclusively, to output coupling networks for hybrid transponders and to hybrid transponders incorporating such networks.
2. Discussion of Prior Art
A hybrid transponder, also known as a shared power amplification module, has been described in the article "An adaptive multiple beam system concept" by S. Egami and M. Kawai in IEEE Journal on Selected Areas in Communications, Volume SAC-5, No. 4, May 1987. FIG. 1 of the accompanying drawings shows a four-way Hybrid Transponder. The four input signals are split and combined in the input network of 90.degree. hybrids in such a manner that all four amplifiers have identical signal levels incident on them. The amplified outputs are combined by the output network of 90.degree. hybrids such that the signal at No. 1 input appears at No. 4 output alone, No. 2 input at No. 3 output, No. 3 input at No. 2 output and No. 4 input at No. 1 output.
The output network must be able to handle high power levels and it is important that signal losses through the output are minimal to maximize overall efficiency. The lowest possible loss medium for microwave transmission is considered to be waveguide. An important requirement in many applications is that the network should maintain amplitude and phase coherence of all the different paths. In other words, the electrical length and loss from any input terminal to any output terminal of the network should be the same. The arrangement of FIG. 1 requires cross-linking between diagonally opposed 90.degree. hybrids of the output network, as shown in FIG. 2 of the accompanying drawings. This would normally necessitate use of bends or cross-overs which add to the size of the network and may cause amplitude and phase mismatch.
A need exists, therefore, for a compact network which provides amplitude and phase coherent interconnections in a compact arrangement.